Maleic anhydride is of significant commercial interest throughout the world and is extensively used in the production of alkyd resins. It is also a versatile intermediate for chemical synthesis. Consequently, large quantities of maleic anhydride are produced each year to satisfy these needs.
In general, catalysts utilized for the oxidation of benzene and C.sub.4 hydrocarbons, such as butene, butane, and butadiene, to maleic anhydride are based upon vanadium and phosphorus. Various metal activators have been used to enhance the phosphorus-vanadium catalyst. The difficulty with the phosphorus-vanadium metal-promoted catalysts is that they tend to deactivate quite quickly. In this connection, U.S. Pat. Nos. 4,020,174, 4,094,816, and 4,089,807 teach that carbon tetrachloride can be used to reactivate the vanadium-phosphorus cometal-promoted catalyst. In U.S. Pat. No. 3,296,282 and U.S. Pat. No. 3,474,041, there is described a method for the regeneration of vanadium-phosphorus oxidation catalysts used in the oxidation of olefins to make maleic anhydride. These references disclose the process of treating the catalyst with a phosphine, phosphite or phosphonate by periodically or continuously passing the phosphorus compound to the reactor, with or without interrupting the olefin feed flow. British Patent Specification No. 1,464,198 teaches regeneration of phosphorus complexes with certain phosphates. These references do not disclose using vanadium-phosphorus catalysts in the presence of about 1 to about 10,000 parts per million of a peroxide to achieve improved catalyst performance and improved oxidation of the hydrocarbon feed to maleic anhydride.
This invention comprises a process for oxidizing benzene or C.sub.4 hydrocarbons such as butane, butene, butadiene, and mixtures thereof to maleic anhydride by contacting it in the presence of oxygen with a vanadium-phosphorus or vanadium-phosphorus cometal catalyst in the presence of about 1 to about 10,000 parts per million by weight of a peroxide based on the total weight of the feed gas stream and for improving the performance of the vanadium-phosphorus catalyst.
In order to maintain the performance of the vanadium-phosphorus or vanadium-phosphorus cometal catalyst an alkyl ester of ortho phosphorus acid, i.e., an alkyl phosphate, is generally added in the amount of about 0.1 to about 100,000 parts per million by weight of the reactor feed gas stream. In a preferred process using continuous catalyst regeneration, the amount of alkyl phosphate added is in the range of about 0.1 to about 30 parts per million by weight of the reactor feed stream. Higher concentrations of alkyl phosphate generally above about 30 parts per million by weight are useful in a batch catalyst regeneration process, preferably in a range of about 50 to about 100,000 parts per million by weight of reactor feed gas stream and more preferably about 1000 to about 100,000 parts per million by weight of reactor feed gas stream. The reactivation is conducted at a temperature of about 650.degree. to about 900.degree. F. The alkyl phosphate in a water medium comprising about 0.001 to about 90 weight percent, more preferably about 0.01 to about 50 weight percent, of the solution is contacted with the feed gas stream flowing to the reactor. If desired, the water and alkyl phosphate may be added separately to the feed gas stream instead of as a solution. Alternatively, the alkyl phosphate and water may be added directly to the butane feed prior to the mixing of the butane and air reactants. The total amount of water added to the reactor feed gas stream is up to 40,000 parts per million by weight and preferably in the range of about 1000 to about 40,000 parts per million by weight.
The type of catalyst useful for this process comprises a phosphorus-vanadium mixed oxide or a phosphorus-vanadium mixed oxide promoted by metals such as molybdemun, zinc, tungsten, uranium, tin, bismuth, titanium, niobium or cobalt. The atomic ratio of the vanadium to phosphorus can suitably be in the range of 0.5:1 to 1.25:1.0. The total atomic ratio of vanadium to phosphorus advantageously is in the range of 0.75:1 to 1:1. It is preferred that the total atomic ratio of molybdenum, zinc, tungsten, uranium, tin, bismuth, titanium, niobium or cobalt to vanadium should be in the range of 0.001:1 to 0.2:1.
The cometal, such as molybdenum, zinc, tungsten, uranium, bismuth, titanium, antimony, niobium, cobalt or tin may be added as a compound together with vanadium, or seprately introduced during the preparation of the catalyst. Suitable cometal compounds comprise their oxides and soluble salts. Suitable molybdenum compounds comprise molybdenum oxide and most soluble molybdenum salts. If it is desired to improve physical properties of the catalysts, they may be treated with an inert support, for example, alumina, titania, silicon carbide, kieselguhr, pumice or silica. The catalyst may be reinforced with such materials at any stage in its preparation.
The oxidation of butane to maleic anhydride may be accomplished by contacting n-butane in low concentration in oxygen with the described catalyst. Air is entirely satisfactory as a source of oxygen, but synthetic mixtures of oxygen and diluent gases such as nitrogen may also be employed. Air enriched with oxygen may be used.
The gaseous feed stream to the oxidation reactors will normally contain air and about 0.2 to about 2.5 mole percent of the hydrocarbon such as benzene, butane, butene or butadiene. About 0.8 to about 1.8 mole percent of the hydrocarbon is satisfactory for optimum yield of maleic anhydride for the process of this invention. Although higher concentrations may be employed, explosive hazards may be encountered. Lower concentrations of the hydrocarbon feedstock, less than about one percent, of course, will reduce the productivity obtained at equivalent flow rates and, thus, are not normally employed for economic reasons. The flow rate of the gaseous stream through the reactor may be varied within rather wide limits, but the preferred range of operations is at the rate of about 100 to about 4000 cc of feed per cc of reactor volume per hour and more preferably about 1000 to about 2400 cc of feed per cc of reactor volume per hour. Lower flow rates make the butane oxidation process uneconomical. A catalyst should be effective at flow rates of about 1200 to about 2400 cc of hydrocarbon feed per cc of reactor volume per hour. There are catalysts which show good promise but when subjected to the hourly space velocity designated above show very poor yields. Residence times of the total gas stream will normally be less than about four seconds, more preferably less than about one second. The flow rates and residence times are calculated at standard conditions of 760 mm of mercury and at 0.degree. C.
A variety of reactors will be found to be quite satisfactory including multiple tube heat exchanger-type reactors. The tops of such reactors may vary in diameter from about one-quarter inch to about three inches, and the length may be varied from about three to about sixteen or more feet. The oxidation reaction is an exothermic reaction and, therefore, relatively close control of the reaction temperatures should be maintained. It is desirable to have the reactors at a relatively constant temperature and some medium is needed to conduct heat from the reactors, such as lead and the like, but it has been found that eutectic salt baths are completely satisfactory. One such salt bath is a sodium nitrate, sodium nitrite, and potassium nitrate eutectic constant temperature mixture. An additional method of temperature control is to use a metal block reactor whereby the metals surrounding the tube act as a temperature regulating body. As will be recognized by one skilled in the art, the heat exchanger medium may be kept at the proper temperature by heat exchangers and the like. The reactor or reaction tubes may be iron, stainless steel, carbon steel, nickel, glass tubes such as vycor, and the like. Both carbon steel and nickel tubes have excellent long life under the conditions of the reaction described herein. Normally, the reactors contain a preheat zone containing an inert material such as one-quarter-inch Alundum pellets, inert ceramic balls, nickel balls, or chips and the like present at about one-half to one-tenth the volume of the active catalyst present.
The reactors that are suitable for the process of this invention may be fixed bed reactors wherein the catalyst remains stantionary within the reactor, they may be fluidized bed reactors wherein the catalyst particles are continually moving within the reactor, and they may be transport or so-called moving bed reactors. In the transport bed rectors, the catalyst is moved, preferably continuously, from within the reaction zone to a second zone where the catalyst is activated. The activated catalyst is then moved back into the reaction zone for reaction with the hydrocarbon feed. By utilizing a transport bed reactor, the need for incorporating oxygen or air in the reactor feed gas stream is greatly reduced or eliminated. In this embodiment of the process, the catalyst is exposed to oxygen away from the reactor zone and it is the oxygenated catalyst that effects the oxidation reaction for converting the benzene or C.sub.4 hydrocarbon to maleic anhydride.
The temperature of reaction may be varied within some limits, but normally the reaction should be conducted at a temperature within a rather critical range. The oxidation reaction is exothermic and once reaction is underway, the main purpose of the salt bath or other medium is to conduct heat away from the walls of the reactor and control the reaction. Better operations are normally obtained when the reaction temperature employed is no greater than about 20.degree. to about 50.degree. F. above the salt bath temperature. The temperature of the reactor, of course, will also depend to some extent upon the size of the reactor and hydrocarbon feedstock concentration.
The reaction may be conducted at atmospheric, superatmospheric or below atmospheric pressure. The exit pressure will be at least slightly higher than the ambient pressure to ensure a positive flow from the reactor. The pressure of the inert gases must be sufficiently higher to overcome the pressure drop through the reactor. The preferred pressure is from about 10 psig to about 100 psig.
We have now found that the addition of a peroxide to the reactor feed gas stream is highly beneficial for oxidation of benzene or a C.sub.4 hydrocarbon feedstock to maleic anhydride using a phosphorus-vanadium or phosphorus-vanadium cometal catalyst. The use of the peroxide results in a lower reactor salt bath temperature for a given feed conversion. The lower salt bath temperature will result in prolonged catalyst lifetime, i.e., the time it takes before the catalyst deactivates and requires replacement.
Due to the expense of manufacturing the phosphorus-vanadium or phosphorus-vanadium cometal catalyst and the expenses associated with loading new catalyst into the maleic anhydride reactor, it is highly advantageous for the catalyst to have as long a useful lifetime as possible. Therefore, any means, such as the process of the instant invention, for prolonging the lifetime of the catalyst is highly desirable.
The amount of peroxide required is in the range of from about 1 to about 10,000 parts per million by weight based on the total reactor feed gas stream. Preferably, said peroxide is present in said reactor feedstream in an amount of from about 5 to about 1000 parts per million by weight.
While hydrogen peroxide is the most preferred peroxide due to availability, expense and ease of handling, other peroxides are also suitable. These other peroxides are the organic peroxides and generally include alkyl, alkenyl, aromatic, and alkyl-aromatic peroxides, hydroperoxides, peroxy acids, peroxy esters and diacylperoxides. Examples of useful organic peroxides are those peroxides selected from the group consisting of ##STR1## wherein R.sub.1 and R.sub.2 are the same or different and are selected from H or C.sub.1 to C.sub.10 alkyl, alkenyl, aromatic or alkyl aromatic hydrocarbyl substituents. The hydroperoxides are preferred over the other organic peroxides due to availability and costs. The hydroperoxides have the general formula R.sub.1 --O--O--R.sub.2 where R.sub.2 is H. R.sub.1 is a hydrocarbon substituent as described above. Particularly preferred hydroperoxides include, for example, t-butyl hydroperoxide, amyl hydroperoxide, cyclohexyl hydroperoxide, methyl-, ethyl-, isopropyl-, n-butyl-, sec-butyl-, and tert-amyl-hydroperoxide. Mixtures of the peroxides described above are also useful.
In addition to lowering the salt bath temperature for the maleic anhydride oxidation reaction, the addition of a peroxide such as hydrogen peroxide provides other benefits. For example, hydrogen peroxide is known to decompose thermally to water and oxygen. As already mentioned, water is added to the phsophorus-vanadium cometal catalyst along with the alkyl phosphate. Therefore, when hydrogen peroxide decomposes, additional water is supplied to the catalyst. Also, the decomposition of the hydrogen peroxide supplies additional oxygen required for the oxidation reaction.
The peroxide can be added to the feed gas stream by any one of a number of methods. The method of addition used is not crucial to the invention. However, it may be added dropwise to the reactor feed gas stream or sprayed or added as a vapor to the reactor feed gas stream.
Maleic anhydride may be recovered by a number of ways well known to those skilled in the art. For example, the recovery may be by direct condensation or by absorption in suitable media.
In summary, the disclosed process comprises the vapor-phase oxidation of benzene or C.sub.4 hydrocarbon feedstock in a reactor to form maleic anhydride in which benzene or the C.sub.4 hydrocarbon is contacted in the presence of air or molecular oxygen or other oxygen containing gas with a catalyst comprising vanadium and phosphorus wherein at least one peroxide is added to the reactor feed gas stream in an amount of from about 1.0 part per million to about 10,000 parts per million by weight of the total reactor feed gas stream.
The following examples will serve to provide a fuller understanding of the invention, but it is to be understood that these examples are given for illustrative purposes only and should not be interpreted as limiting the invention in any way.